Packet-based communication technologies, such as ITU G.hn, HomePNA, HomePlug® AV and Multimedia over Coax Alliance (MoCA), are becoming increasingly important due to the significant advances in broadband networks capable of delivering multimedia-rich IP-based services. As the demand for such services increases, service providers are looking for improved ways to distribute digital content within consumers' homes. The aforementioned technologies are designed specifically for this purpose.
Digital subscriber line (DSL) technology may be used to transform an ordinary telephone line (e.g., copper wire twisted-pair) into a broadband communication link. DSL technology achieves this by sending signals over the telephone line in previously unused high frequencies.
Over the years, DSL technology has evolved into a family of specific, standardized implementations. These various implementations, which include among others Asymmetric Digital Subscriber Line (ADSL), ADSL2, ADSL2+, Very high speed DSL (VDSL), VDSL2, G.Lite, and High bit rate Digital Subscriber Line (HDSL), offer a variety of transmission speeds and transmission distances. It is common to refer to the various DSL implementations that have evolved over the years collectively as xDSL.
Wireless communication technology may be used in connection with DSL technology to increase the general mobility of devices that are used to access a broadband communication link. A common wireless communication technology used in connection with DSL technology and other broadband services includes the various IEEE 802.11 standards for wireless LANs.
Various xDSL implementations typically employ either Carrier-less Amplitude and Phase (CAP) modulation or discrete multi-tone (DMT) modulation. CAP modulation is a variation of quadrature amplitude modulation (QAM). CAP modulation produces the same form of signal as QAM without requiring in-phase and quadrature carrier components. DMT modulation is a modulation method in which the available bandwidth of a communication channel is divided into numerous orthogonal sub-channels. Sub-channel is also referred to as sub-carrier or tone. Each tone of a DMT communication system is capable of acting as a communications sub-channel that carries information between a transmitter and a receiver. Multicarrier systems that make use of DMT modulation may also be referred to as systems that employ Orthogonal Frequency Division Multiplexing (OFDM). OFDM is also the modulation method employed by many wireless communication technologies, including ITU G.hn, G.hnem, HomePlug® AV, MoCA, IEEE 802.11 and IEEE 802.16. Technologies implementing DMT, OFDM and other multicarrier modulation schemes are generalized herein as being multicarrier systems.
In various multicarrier modulation schemes, the number of data bits or the amount of information that a tone carries may vary from tone to tone, and it depends on the relative power of the data-bearing signal compared to the power of an interfering signal and/or background noise on that particular tone. One measure of the quality of a signal carried by a tone is the ratio of the received signal strength (power) over the noise strength in the frequency range of operation, or the Signal-to-Noise Ratio (SNR). Generally, a high SNR results in high signal quality being carried by a tone. Another measure of signal quality is bit error ratio (BER) for a given tone. In addition, another potential measure of signal quality is the likelihood of interferers at one or more frequencies.
Communication devices implementing multicarrier modulation schemes often go through a brief training period, also known as channel discovery phase or sounding phase before data communications begin. During the training period, test signals may be transmitted to effectively test the quality of the communication medium at various frequencies. Generally the medium quality is determined by measuring the SNR on each of the tones. The number of bits allocated to each tone is determined based in large part on the channel measurements such as detected training signals. Upon completion of the training period, most multicarrier modulation schemes implement a Bit Allocation Table (BAT) that specifies the number of bits modulated on each tone. In some cases it might be beneficial to load the same number of bits for all tones, which does not require BAT, but still can be considered as a special case of BAT.
After the training period and the establishment of a BAT used between communicating transceivers, the transmission medium often encounters changes that may affect the communication of information at the allocated bits on some of the tones. A known channel adaptation or channel tracking process may be used to replace a BAT used between communicating transceivers with a new BAT that includes bit allocation that sufficiently matches a current state of the transmission medium. However, the known channel adaptation processes may not provide effective channel adaptation for certain systems. For example, one or more such processes may have the drawback of taking a relatively large amount of time, which may lead to brief service interruptions under rapidly changing channel conditions.